A semiconductor device such as an LED and a power module is provided with a structure in which a semiconductor element is bonded onto a circuit layer formed from a conductive material.
In a large-power control power semiconductor element that is used to control wind power generation, an electric vehicle, a hybrid car, and the like, the amount of heat generation is great. According to this, as a substrate on which the large-power control power semiconductor element is mounted, for example, a power module substrate including a ceramic substrate formed from aluminum nitride (AlN), alumina (Al2O3), and the like, and a circuit layer formed by bonding a metal plate with excellent conductivity on one surface of the ceramic substrate has been widely used in the related art. Furthermore, as the power module substrate, a power module substrate, in which a metal layer is formed on the other surface of the ceramic substrate, is also provided.
For example, a power module disclosed in PTL 1 has a structure including a power module substrate in which a circuit layer and a metal layer which are formed from Al are respectively formed on one surface and the other surface of a ceramic substrate, and a semiconductor element that is bonded onto the circuit layer through a solder material.
In addition, a heat sink is bonded to a metal layer side of the power module substrate to radiate heat, which is transferred from the semiconductor element to the power module substrate side, to an outer side through the heat sink.
However, as is the case with the power module described in PTL 1, in a case where the circuit layer and the metal layer are constituted by Al, an oxide film of Al is formed on a surface, and thus it is difficult to bond the semiconductor element or the heat sink onto the surface with the solder material.
Accordingly, in the related art, for example, as disclosed in PTL 2, after a Ni plating film is formed on the surface of the circuit layer and the metal layer through electroless plating and the like, the semiconductor element or the heat sink is subjected to solider-bonding.
In addition, PTL 3 suggests a technology of bonding the circuit layer and the semiconductor element, and the metal layer and the heat sink, respectively, by using silver oxide paste containing a reducing agent including silver oxide particles and an organic material as an alternative of the solder material.
However, as described in PTL 2, in the power module substrate in which the Ni plating film is formed on the surface of the circuit layer and the metal layer, during bonding of the semiconductor element and the heat sink, a surface of the Ni plating film deteriorates due to oxidation and the like, and thus there is a concern that bonding reliability of the semiconductor element and the heat sink which are bonded through the solder material deteriorates. Here, when bonding between the heat sink and the metal layer is not sufficient, there is a concern that heat resistance increases, and thus heat dissipation characteristics deteriorate. In addition, in a Ni plating process, a masking process may be performed in order prevent problems such as electrolytic corrosion due to formation of the Ni plating in an unnecessary region from occurring. As described above, in a case of performing a plating process after performing the masking process, a great deal of labor is necessary in the process of forming the Ni plating film on the surface of the circuit layer and the surface of the metal layer, and thus there is a problem in that the manufacturing cost of the power module greatly increases.
In addition, as described in PTL 3, in a case of respectively bonding the circuit layer and the semiconductor element, and the metal layer and the heat sink by using the silver oxide paste, bondability between Al and a baked body of the silver oxide paste is poor, and thus it is necessary to form a Ag underlying layer on the surface of the circuit layer and the surface of the metal layer in advance. In a case of forming the Ag underlying layer through plating, there is a problem in that a great deal of labor is necessary similar to Ni plating.
Accordingly, PTL 4 suggests a power module substrate in which the circuit layer and the metal layer are set to have a laminated structure of an Al layer and a Cu layer. In the power module substrate, the Cu layer is disposed on the surface of the circuit layer and the metal layer, and thus it is possible to bond the semiconductor element and the heat sink by using a solder material in a satisfactory manner. As a result, heat resistance in a laminating direction decreases, and thus it is possible to transfer heat, which is generated from the semiconductor element, to the heat sink side in a satisfactory manner.
In addition, PTL 5 suggests a power module substrate with heat sink in which one of the metal layer and the heat sink is constituted by aluminum or an aluminum alloy, the other side is constituted by copper or a copper alloy, and the metal layer and the heat sink are subjected to solid-phase diffusion bonding. In the power module substrate with heat sink, the metal layer and the heat sink are subjected to the solid-phase diffusion bonding, and thus heat resistance is small, and heat dissipation characteristics are excellent.